<!DOCTYPE html>
<html lang="en">
  <head>
    <meta charset="UTF-8" />
    <meta http-equiv="X-UA-Compatible" content="IE=edge" />
    <meta name="viewport" content="width=device-width, initial-scale=1.0" />
    <title>Document</title>
  </head>
  <body>
    <script>
      function banck() {
        let num = prompt('请输入你的总额')
        let num1 = prompt('请输入你的月食宿金额')
        let num2 = prompt('请输入你的月消费金额')
        let num3 = num - num1 - num2
        return num3
      }
      let res = banck()
      alert(`银行卡剩余金额为 ${res} `)
    </script>
  </body>
</html>
